This invention relates to a glow plug utilized to preheat the sub- or auxiliary combustion chamber or the main combustion chamber of a diesel engine.
The starting characteristic of a diesel engine is generally poor at low temperature so that it is the common practice to increase the suction temperature by installing a glow plug in the sub-combustion chamber or main combustion chamber and by passing electric current through the glow plug. The glow plug is also used as a source of ignition for improving the starting characteristic of the diesel engine. As the glow plug, so-called sheathed type glow plug has been used in which a helical heating wire made of iron, chromium or nickel or alloys thereof is packed in a heat resistant electric insulating powder contained in a sheath made of heat resistant metal with the helical heating wire connected at one end to the tip of the sheath. The sheathed type glow plug can be used over a long time in high temperature gas in the sub- or main combustion chamber.
With the sheathed type glow plug described above, since the sheath is heated indirectly by the heating wire it takes a long time to increase the temperature of the glow plug to the operating temperature. More particularly, the heat generated by passing electric current through the heating wire is transmitted to the sheath through the heat resistant electric insulating powder, it takes a long time before the sheath becomes red hot. Moreover, the efficiency of heat transmission is low. Thus for example, it takes several tens seconds until the temperature of the plug is increased to 800.degree. C., for example, thus failing to rapidly start the diesel engine.
The temperature rise time can be decreased by passing a large current at the initial stage of energization so as to cause the heating wire to quickly generate heat, but such measure is liable to melt the heating wire or to damage the sheath by high temperature. This also affects the battery and electric circuit and in an extreme case melts a fuse. To prevent these problems, it is necessary to provide a temperature control device for the heating wire which increases the cost of the preheating device.
In recent years, diesel engines are abundantly used for general purpose motor cars, and in order to make the diesel engines to be comparable with gasoline engines it is desired to develope a quick heating type glow plug for improving the starting characteristic.
To meet such requirement, various improvements have been made including the selection of the material for the heating wire, and to improve the temperature rise PG,4 characteristic by supplying a large current glow plug have been developed in which heating wires made of two types of materials having different resistance temperature coefficients are used as disclosed in British Pat. No. 1,376,817 and Japanese Laid Open patent specification No. 109,538 of 1979.
Although the glow plug utilizing heating wires made of two types of materials can quickly raise the temperature, its cost of manufacturing is high and the heat generating characteristic is not yet satisfactory.
As a result of exhaustive investigation, we have found that in the previously developed construction, the heating wires of two types are too closely disposed so that the interval of passing large current at the initial stage is shortened due to the effect of heat.
More particularly, in the prior art glow plug of this type, a heater having a lower resistant temperature coefficient is disposed on the front side, while a heater having a higher resistant temperature coefficient is disposed on the rear side and wherein the resistance variation caused by the temperature variation of the rear side heater is used to supply large current to the front side heater immediately after starting energization so as to rapidly red heat the front side heater. However, since both heaters are closely disposed the temperature of the rear side heater is too quickly raised due to the effect of heat from the front side heater whereby the resistance of the rear side heater increases to limit the current supplied to the front side heater. For this reason, the prior art glow plug of this type has poor heating characteristics and can not manifest rapid heating characteristic.